The applicant has provided an oil filter provided with a relief valve shown in FIG. 4 (refer to Japanese Patent Application No. HEI 11-92540). This oil filter comprises a filter element 1 having an annular structure through which oil passes, a cap 2 having substantially a cylindrical shape and covering an outer periphery of the filter element 1, a holder 3 fitted into the inside of the filter element 1, and a relief valve 8 which operates at a time when the filter element is clogged. In this FIG. 4, a base of the oil filter to be mounted to an engine side is not illustrated.
A valve seat 4 constituting a relief valve 8 is welded to an inner surface of a top portion of the cylindrical cap 2. This valve seat 4 is formed with a claw portion 4b to be engaged with an upper end portion 3a of the holder 3. And a valve body 5 is mounted to the lower surface of the valve seat 4. The valve body 5 is urged against the valve seat 4 by a coil spring 6, which is disposed between the valve body 5 and a spring receiving portion 7 formed on the holder 3.
In an occurrence that the filter element 1 clogs and pressure difference between a dirty side and a clean side is more than a predetermined pressure, the valve body 5 is depressed against a spring force of the coil spring 6. According to this action, the oil passes around the filter element 1 and passes through an oil passage 4a opened to the valve seat 4.